1. Field of the Invention
This invention relates to an anti-skid control system for achieving a satisfactory rotational behavior of the wheels of a motor vehicle during a braking operation. More particularly, this invention pertains to an anti-skid control system which is designed to increase and reduce brake fluid pressure in a desirable manner to prevent the wheels from becoming locked.
Especially, the present invention relates to a method for discriminating noise data from actual wheel velocity data for anti-skid brake control.
2. Description of the Related Art
There have been heretofore proposed a variety of anti-skid control systems wherein, during a braking operation, the brake fluid pressure is reduced to prevent the occurrence of an undesirable wheel lock, and the brake fluid pressure thus reduced is then increased to avoid a undesirable extension of the brake stopping distance. Among such conventional anti-skid control systems is one wherein different pressure buildup rates are stored; a desired one of the stored pressure buildup rates is selected; and brake fluid pressure is increased with the selected buildup rate, for example. To effect the pressure buildup rate selection, a system has been proposed, wherein a solenoid-operated valve adapted to be opened and closed with a relatively high frequency is provided in the brake fluid pressure buildup system; a pulse train generator such as a multivibrator is provided in the drive system for the solenoid-operated valve; and the timing with which the solenoid-operated valve is driven by a pulse train signal derived from the multivibrator is controlled on the basis of a signal representing wheel acceleration (British Patent Specification No. 1305430).
A conventional anti-skid brake control system comprises a wheel velocity detection means which is attached to the vehicle wheel axle and generates pulse signals corresponding to the wheel velocity. The wheel velocity is calculated based on the period of one cycle of the pulse signal and the result of the calculation is used to control the brake pressure.
The pulse signals from the wheel velocity detector are disturbed by the vibration from tires or by electric noise. An example of the output signal of the wheel velocity detector is represented in FIG. 4. As shown in the graph of FIG. 4, the wheel velocity may be mistakenly discriminated as an acceleration state, although actually in a deceleration state, due to the noise N.sub.A. Conversely, the wheel velocity may be discriminated as a deceleration state although it is actually in an acceleration state, due to the noise N.sub.B. Such a misjudgement will cause a malfunction of the anti-skid brake control system.
FIG. 5 shows a graph (i) of pulse signals from the wheel speed sensor and a graph (ii) of the output of the counter (cycle period detector) which represents pulses each having a width corresponding to one cycle of the pulse signal of graph (i). The above mentioned noise appears in the graph (i), which generates an unnecessary signal (n) having a period t.sub.3, thus causing a malfunction of the system.
To avoid such a malfunction, an integral circuit has been used to modify and average the velocity curve, as represented by the dashed line in FIG. 4. However, the integral circuit can not effectively avoid the affect of noise having a large period. Also, a detection timing of the peaks of the wave signal is delayed due to the usage of the integral circuit, which degrades the reliability and stability of the anti-skid brake system.